The salient features of this proposal are: The study of those structural features of human erythropoietin that may be directly involved with its biological activity on hemopoietic precursor cells; the study of the primary structure of the active site; the determination of oligosaccharide side chain structure; the isolation and characterization of specific cellular receptors for erythropoietin from cells infected with the anemia strain of the Friend virus; the extension of receptor studies to normal mouse cells and to apply the methods developed for that purpose to the study of human cellular receptors. Once isolated the erythropoietin receptor will be cloned and the gene for the receptor used to test a hypothesis regarding erythroid cell differentiation. Mouse erythropoietin will also be cloned and the DNA used for the study of the regulation of expression of the erythropoietin gene as well as for the study of erythropoietin biogenesis and secretion. Another aim is to develop a solid-phase radioimmunoassay using monoclonal anti-epo with the intention of improving the sensitivity of analyis by an order of magnitude. The same monoclonal anti-epo will be used as the basis for an improved immunoaffinity purification method.